1. Field of the Invention
This invention relates to a method of detecting a face image in digital images, and a system and a computer program for carrying out the methods.
2. Description of the Related Art
There has been carried out to investigate a color distribution of the face of a human image in a snapshot picture taken by a digital still camera and to correct a color of the skin and/or to recognize a human in a digital image taken by a digital video camera of a watching system. In such a case, since it is necessary to detect face areas corresponding to the human face in the digital image, there have been proposed various methods of detecting a face image including a face in a digital image.
For example, a method in which a plurality of fraction images are cut out from an object image in a plurality of different positions thereof and whether the fraction images are an image including a face is determined, whereby a face image on an object digital images is detected can be listed.
Further, when a face image whose inclination cannot be identified is to be detected, it is necessary to determine while changing the position and the inclination of the face to be detected on the object image in sequence whether the fraction image in the position is a face image including a face of the inclination by the inclination of the face to be detected.
In order to determine whether the fraction image is a face image, for instance, use of technic of template matching or a technic employing a determiner module which has learned the feature of face by machine learning (a technic disclosed, for instance, in “Fast Omni-Directional Face Detection”, Shihong Lao et al., MIRU 2004, pp. II271-II276 or U.S. Patent Application publication No. 20050100195) is conceivable. But in anyone of the above methods, for the determination, it is general to calculate an index representing the probability that the fraction image is a face image on the basis of the image pattern of the fraction image and to determine that the fraction image is a face image when the index exceeds a predetermined threshold value.
However, in the above face detecting methods employing a technic which determines that the image is a face image by comparison of the index based on the image pattern with a threshold value, there has been a problem that escape of a face image is apt to occur when there is an evenness in brightness or the contrast in the object image and the object image is low in image quality, or when a face in the object image is partly blocked by another object and features of the face cannot be sufficiently grasped, and that a non-face image is mistaken for the face image when a non-face image which resembles to the face image in the image pattern accidentally exists near the face image on the object image.
Further, in a face detecting method where while changing the position and the inclination of the face to be detected on the object image, whether the fraction image in the position is a face image including a face of the inclination is determined by the inclination of the face to be detected in sequence, the amount of processing required to the determination becomes huge since the determination must be carried out the number of times corresponding to the number of combinations of the positions and inclinations of the face to be detected, and this fact is a factor of preventing the face detection from being carried out at high speed.